1. Field of the Invention
The present invention relates to a polymer or polymer composite membrane having through-thickness micropores and a method of preparing the same, and particularly, to a polymer or polymer composite membrane having through-thickness micropores and a method of preparing the same, in which the polymer or polymer composite membrane has a pore structure in which the micropores are aligned in a mesh structure in a thickness direction by unidirectional freezing in a thickness direction of a solvent, has improved permeability in a thickness direction due to the presence of the through-thickness micropores and excellent uniformity in size of the micropores and wall thickness between the pores, and thus may be used in a porous membrane substrate, microfiltration membrane, etc.
2. Discussion of Related Art
As a functional polymer suitable for effectively driving a membrane, a polymer having excellent thermal stability, chemical stability, oxidation stability and hydrophobicity is generally required. Currently used polymers satisfying these conditions include fluorine-based polymers having a C—F bond and hydrophobic polymers such as polyethylene-, polypropylene-, epoxy-, and polyimide-based polymers. The fluorine-based polymers are physically strong and highly chemically stable due to a high binding strength between fluorine and carbon, and thus have very good durability. As a representative fluorine-based polymer containing sulfone, there is Nafion®, whose chemical name is perfluorosulfonic acid.
Meanwhile, as a polymer having thermal stability, chemical and electrochemical stability and a mechanical strength, polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) are widely known. PVDF is relatively simply processed, and its characteristics and performances have been proved since it is already used as a binder in a lithium secondary battery. In addition, PVDF has compatibility with ceramic powder. Examples of applying combinations of PVDF and an inorganic powder to a battery are disclosed in U.S. Pat. Nos. 5,296,318 and 5,643,689, and International Publication Nos. WO 99/44245.
In Korea, technology for mainly preparing a PVDF separation membrane as a porous separation membrane having excellent chemical resistance and durability has been developed and is thus currently used. In the case of PTFE, it has been reported that a planar membrane-type PTFE filter was developed, but a commercial product has not been released. Globally, there are various examples of commercializing the PTFE polymer as a porous separation membrane, but Sumitomo, Japan is the only company to prepare and sell a PTFE membrane in the form of a hollow fiber membrane.
Since a material for the PTFE membrane mentioned above may not be processed by a conventional method for preparing the PTFE filter such as a solvent/non-solvent conversion method or a thermally induced phase separation method using melt spinning, understanding of the material and research on processing factor technology (including extrusion, sintering, stretching, etc.) are at a very poor level, and the technology has not been sufficiently accumulated.
A representative method of preparing a membrane is a method of preparing a transparent porous membrane from a polymer and an inorganic powder using a low boiling point solvent and a high boiling point solvent having a high volatility temperature that is soluble in water by forming micropores by extracting the solvent having a high volatility temperature that is soluble in water.
According to the above-mentioned method, since the preparation process is complicated, and prepared pores are not formed through the membrane in a thickness direction, connection of amorphous pores is induced to increase overall porosity. In addition, since the prepared membrane should be dried at room temperature, the preparation of the membrane takes a long time, and the extraction of the high boiling point solvent contained in the membrane also takes a long time, which means that the method is difficult to apply on a commercial scale. When the membrane is used for an energy device, a remaining solvent may have a negative effect on battery performance, and when porosity characteristics such as a forming direction of the pores and a pore size are not suitably controlled, the method is also difficult to apply in terms of permeability.
Accordingly, there is a demand for a simple and economical method of easily controlling a size, structure and porosity of a pore, thereby mitigating these problems and determining characteristics of a membrane. Particularly, to effectively improve and control permeability, it is necessary to form micropores through a membrane having low tortuosity in a thickness direction. In addition, a method of forming the micropores should not be an inefficient method, for example, of punching the membrane using a template or laser beam to form each micropore in the thickness direction.